Hot air infrared thermography is one form of nondestructive evaluation of parts. In this method, hot air may be directed through an internal passage in a test piece in one or more pulses. During or after this pulse, an infrared sensor captures infrared emission information from an outside surface of the test piece. The infrared sensor is able to detect small variations in the infrared emission information indicative of different temperatures on the outside surface of the test piece. The infrared emission information can be used to generate a thermal image of the test piece in which the thermal variations across the surface of the test piece are visible. Areas of the outside surface nearer the internal passage will generally be hotter and therefore be visible in the thermographic image via a greater intensity. As a result the thermographic images can be used to evaluate internal passages of the test piece to determine, for example, its cooling efficiency, without any need to destroy the test piece.
Variations in testing conditions can lead to differences between thermographic images of the same test piece. For example, often a single test piece is subjected to several pulses of hot air in order to gain sufficient data for an accurate reading. The temperature of the hot air for each pulse may change due to the length of hoses, outside temperature, and compressor etc. Further, with each pulse the test piece heats up, and with each delay between pulses the test piece cools down, and thus the emission intensity of the test piece may change with each pulse. While a relative temperature distribution of the test piece will show in each of the thermographic images, an intensity level of the image may vary from image to image. This variation in intensity may be particularly prevalent near a cooling passage, which will heat up more quickly than the surrounding material. As a result, the cooling passages may vary in intensity from one image to the next relative to the surrounding material. This variation in intensity makes it difficult to detect certain partially blocked passages and measure wall thicknesses etc. Further, less accurate image data makes it difficult to compare thermal efficiency of different blade designs.
In addition, with no reference to the input air temperature, comparing multiple test pieces to each other on the same temperature scale difficult. Consequently, there is room for improvement in the art.